Maize (corn) is one of the most diverse grain crops present in nature, comprising a number of different types, which are generally classified by characteristics of their kernel endosperm. The most common types of corn include flint, flour, dent, pop, sweet, waxy and pod. The physical appearance of each kernel type is determined by its endosperm pattern, quality and quantity.
Sweet corn is a corn plant classified as Zea mays, var. rugosa, and has white, yellow or bi-colored kernels that are sweet when they are in the immature milky stage as a result of having a high sugar content (i.e., sucrose content). Higher levels of sugar or sucrose in the sweet corn kernels result in a lower osmotic potential, causing greater water uptake into the kernels. Sweet corn is typically eaten by human beings as a vegetable, either directly from the maize cob, or by having the sweet kernels removed from the cob, and is a major vegetable crop grown all over the world primarily for fresh consumption, rather than as animal feed or for flour production.
Sweet corn occurs as a spontaneous mutation in field corn and can be the result of naturally-occurring mutations in one or more genes that control conversion of sucrose to starch inside the endosperm of the corn kernel. Unlike field corn varieties, which are intended for livestock and are typically harvested when the kernels are dry and fully mature (at the dent stage), sweet corn is typically picked when it is immature (at the milk stage), and eaten as a vegetable, rather than as a grain. Because the process of maturation involves converting sucrose into starch, sweet corn typically stores poorly and must be eaten in a fresh, canned or frozen manner before the kernels become tough and/or starchy. Following harvest, or if left on the stalk too long, sucrose in standard sweet corn becomes rapidly converted to starch. Kernels can lose as much as 50% of their sucrose at room temperature at around 24 hours after harvest.
Open pollinated (non-hybrid) varieties of white sweet corn started to become widely available in the United States in the 19th century. Two of the most enduring varieties, which are still available today, are Country Gentleman (a Shoepeg corn with small, white kernels in irregular rows) and Stowell's Evergreen. Sweet corn production in the 20th century was influenced by the following key developments: (i) hybridization, which allowed for more uniform maturity, improved quality and disease resistance; and (ii) identification of separate gene mutations responsible for sweetness in corn, and the ability to breed varieties based on these characteristics, for example: su1 (sugary); se1 (sugary enhanced); and sh2 (shrunken-2). There are currently hundreds of varieties of sweet corn, with more varieties continuously being developed.
The sh2 gene encodes the ADP-glucose pyrophosphorylase (AGPase) large subunit, gene ID GRMZM2G429899 located on Chr3 map position 216,414,684. The small subunit of AGPase is encoded by Brittle2 (Bt2). AGPase catalyzes the reversible synthesis of ADP-glucose and pyrophosphate from ATP and glucose-1-phosphate and is one of the main regulatory steps in the biosynthesis of starch in plants. A mutation in the sh2 gene called sh2-R results in maize kernels that have greatly reduced starch levels and increased sugar resulting from reduced levels of the enzyme. In addition, sh2-R mutants exhibit reduced seed germination and seedling vigor, as well as reduced yield. Thus, in some settings there is a need in the art for methods for rapid identification of the sh2-R mutation in germplasm so that the mutation can be reduced or eliminated from a breeding population. The present invention provides methods and compositions that address this need.